I have been enjoying my Mainline for almost two years. I built the SEX first, then Mainline, then Crackatwoa
https://forum.bottlehead.com/index.php?topic=14324.msg130465#msg130465, then Crack
https://forum.bottlehead.com/index.php?topic=14560.msg132369#msg132369. They're all great kits.
There is an intermittent problem with one channel on my Mainline, no doubt a bad joint or nicked wire. I believe it is something to do with flexing the wires when lowering the C4S board onto the standoffs. I tried reflowing a few joints, but the problem persists.... intermittently.
Time for a rebuild and I'll take the opportunity to simplify to my needs and customise a little.
I am not aiming for sonic uplift and I haven't listened to the original build recently, so I have no hope of comparing the sound of the old and the new.
I have a personal preference for Crackatwoa with my high impedance headphones, so the Mainline rebuild will be low impedance only. I only use a single input, so I'll drop the second. I only use the balanced output, so I'll drop the unbalanced. I use a combo IEC inlet/switch, so out goes the separate on/off switch. I think I've cut around 40 joints from the rebuild.
Also on the wish list:
- a better way to support the big caps
- hidden fasteners, undermounted components
- ability to set the bias from the top side
- mounts for the C4S and Bias boards - no wire squishing
- attenuation via a single knob
- custom end bell
A few 3D printed parts in polycarbonate (for heat handling), a fresh 4mm anodised top plate (allows for some recessing) and some conductive epoxy* later and I'm almost ready to start the wireup, if the weather allows me to paint the transformer parts.
*As mentioned in my Crack post the standoff/studs have too great a resistance to ground when glued in by the supplier (Front Panel Express). If you plan on using any of the mounts as a path to ground you need to install them yourself. For my Crack I used Loctite and it worked very well. This time I'm trying some silver impregnated two-part epoxy. It had better work fantastically well to justify the price. I'll let you know. For the studs that are not ground points I'll use JB Weld.
I am trying an asymmetric layout, with the power on one side and the channels on the other. Perhaps a gamble that there won't be any interference between the channels. Fingers crossed. The blank space on the right hand side won't be to everyone's taste. Having spent a lot of time on this layout I have an even greater respect for the Bottlehead team designing and bringing these products to us.
To get the BIAS to the top side of the plate I'm using the same brand/spec 2k trim potentiometers as the original, but in a different form - BOURNS 3059Y-1-202LF. There are a number of layouts available for these trimpots. This version comes with mounting holes, making it easy to design a 3D printed mount to suit.
If you happen to be using Front Panel Designer to specify a scale to be laser engraved for a Goldpoint V47 I can tell you they have a 344 degree sweep. I set my start angle to 172 degrees and my end angle to -172 degrees, but you can specify any 344 degree sweep depending on where you want the zero attenuation mark to be. My scale lines up really well with each of the 47 x 1.5 dB steps.
Might take me a couple of weeks to complete the build, I'll post again when I'm finished.
